In the industry related to fabrication of thin plastic sheet several quality control problems can be reduced by the invention claimed herein. It is the current practice to extrude molten plastic through a die which is so constructed as to produce a tubular extrusion. The tubular extrusion is stretched and its walls made thin by varying methods, the most common of which is the application of air pressure within the tubular extrusion. All means of thinning the tube walls are considered equilavent for the purposes of this patent. This is performed as a continuous process and results in the formation of a tubular section. After cooling the tubular section, it is cut into two sections parallel to the tube axis and these sections are edge trimmed and put onto rolls for final market sale.
Current technology of die and cooling apparatus construction often results in a tubular section which has substantial thickness variations. These relative variations are more or less consistent when viewed over long sections of the tubular extrusion. Since a thick or thin section in the resulting sheet will result in a corresponding hill or valley on the final product roll which is not acceptable for most final uses of plastic sheets, various complex methods of obtaining distribution of the variations over the final roll have been developed. The most common are the rotation of the die or the rotation of the take-off assembly through 360.degree.. Either method results in cutting strips in a helical path relative to the extruded tubular section. The major drawback of these systems is the complex machinery involved which is expensive and requires frequent maintenance.
This invention incorporates several improvements which improve the versatility and operation of the process and avoid the several drawbacks of the existing technology.